The present inventions relates to cylindrical lenses as taught in U.S. Pat. Nos. 5,080,706; 5,081,639; 5,155,631; and 5,181,224. The preceding United States patents are herewith incorporated by reference.
Each of the incorporated references teaches a cylindrical lens, and more particularly a cylindrical microlens. As used herein a cylindrical microlens is defined as a cylindrical lens wherein measurements of the X and Y axes thereof are in the range of approximately 50 microns to 1000 microns. U.S. Pat. No. 5,080,706 provides a diffraction limited, high numerical aperture microlens (a fast cylindrical microlens) which is particularly suitable for use in the CIRCULASER.TM. laser diode available from Blue Sky Research of Santa Cruz, Calif. As taught therein, for many applications the curves of the cylindrical lens may be of specific shapes quite different from circular curves or flat planes. The required shape, while it might be flat or circular, could also incorporate a non-circular curve such as an elliptic or hyperbolic. In other words, cylindrical lenses taught in the incorporated references may be formed with a variety of curved surfaces, and the exact shape chosen therefor is highly dependent upon the optical application in which the microlens is intended to be used.
Diffraction limited surfaces can be manufactured on large scale cylindrical optics (i.e., greater than 5 mm) with low numerical apertures (0.1 or less) by conventional grinding and polishing techniques. However conventional grinding and polishing techniques are unable to produce optical quality cylindrical lenses of higher numerical aperture (N.A.). For small microlenses (i.e., less than or equal to 1 mm) other techniques have been developed. Microlenses have been manufactured using photosensitive glass, graded index glass, and computer-generated diffractive optics or kinoforms. None of these techniques has been able to produce a diffraction limited cylindrical lens with high numerical aperture.
In fabricating microlenses from photosensitive glass, a mask is first deposited on the glass, and the material outside the desired lens is exposed to light. When the glass is subsequently heated, the exposed material expands its volume, and the unexposed lens region is compressed. The compression causes the lens region to bulge, forming a simple lens.
Graded index microlenses are formed by diffusing index-changing material into glass. The diffusion process yields an index of refraction that varies smoothly from the lens center to the edge. The graded index focuses the light much as a conventional lens does.
In a binary diffractive optic or computer-generated kinoform, the surface of a glass plate is etched according to a pattern generated by computer. The etched surface is designed to diffract light to a focal point, so that it performs like a conventional lens.
Cylindrical microlenses fabricated from photosensitive glass and graded index planar microlenses can be produced inexpensively in quantity, but these single optical elements are limited to numerical apertures of 0.25 to 0.32, and furthermore they cannot be corrected for spherical aberration. Diffractive optic kinoforms can be corrected for aberrations, but efficient kinoform lenses with numerical apertures approaching 0.5 require the use of sub-quarter-micron lithography, which is currently beyond the state of the art.
Optical fibers with a circular cross-section have been used for cylindrical lenses. Optical fiber is inexpensive and readily available. However, circular optical fibers are not corrected for spherical aberration; i.e., such optical fibers are not diffraction limited.
Cylindrical microlenses have been utilized for integrated optics, for focusing laser diode bars, and also to shape the beam of single-mode laser diodes, as in the previously discussed CircuLaser.TM.. In integrated optics, carefully designed cylindrical microlenses efficiently and conveniently couple light into or out of narrow waveguides, or any narrow slit.
The incorporated references disclose cylindrical microlenses manufactured by "pulling" a preform in similar fashion to the manner in which optical fibers are pulled. The preform itself may be formed by a number of methodologies, but the most commonly utilized technologies for such manufacture are grinding the preform from a billet of optical glass and casting the preform in a mold from a mass of molten optical glass.
The incorporated references do not address the problem of light scattering caused by tool marks left in the preform as a result of the manufacturing process. Hence to effect the manufacture of preforms, one skilled in art would be inclined to utilize longitudinal grinding of either the preform itself or of the mold from which the preform is cast.
Having reference to FIG. 1, a first conventional grinding technology consists of grinding at least one non-flat surface, 10, along glass preform, or boule, 1 in the longitudinal direction, 5. According to this methodology, a substantially flat grinding wheel 20, is mounted on a numerically controlled universal grinder, such as the Model 1632 CNC cylindrical grinder with an optional CNC programmable workhead for non-round grinding, available from Weldon Machine Tool of York, Pa. In this manner, the lens is ground by a succession of longitudinal passes which ultimately form the desired lens face profiles.
A second prior fabrication methodology is shown in FIG. 2. Having reference to that figure, a non-flat grinding wheel 21 is mounted in the previously discussed grinding apparatus in order to grind surface 10 into preform 1. In this embodiment, the negative of desired lens profile is first formed as surface 22 of grinding wheel 21. As wheel 21 is applied to preform 1, again in the longitudinal direction, surface 22 of grinding wheel 21 imparts the profile of lens surface 10 to preform 1. In this case, CNC grinding is not required, as the lens is ground by at least one, and preferably a succession of longitudinal passes which ultimately form the desired lens face profiles.
All grinding of lenses introduces tool marks. Each of the previously discussed longitudinal grinding methodologies induce tool marks which are formed along the lens's longitudinal axis. These longitudinal tool marks have been found to be persistent: they remain even after the lens is heated and pulled to its final size. These relics of longitudinal grinding cause scattering and reduced optical efficiency. In extreme cases, they can cause aberrations to the beam formed by the lens, or shadows in the beam. Moreover, the application of longitudinal grinding to molds wherefrom preforms are cast presents similar limitations. Longitudinal grinding of the molds results in longitudinal tool marks which transfer to the resultant preform.
Transversely grinding the preform, or the mold from which the preform is cast, would eliminate these longitudinal tool marks. However, such transverse grinding of cylindrical lenses, their preforms, or the molds from which cast introduces corresponding transverse tool marks in the lens which have similar detrimental effects on optical performance as the previously discussed transverse tool marks. Moreover, the transverse grinding of cylindrical lenses is well known to have several additional deleterious effects. First, the actual number of tool marks, per unit length of lens, increases with transverse grinding. Second, transverse grinding of optical lenses is somewhat more limited in the shapes which are attainable using this methodology. Such shapes can generally be limited to convex shapes and relatively long-radii concave surfaces. Finally, transverse grinding is more expensive than longitudinal grinding.
What is required then, is a methodology for forming a microlens from a preform wherein the resultant microlens is without tool marks. Specifically, what is needed is a methodology for forming microlenses by first forming a preform without longitudinally grinding it, or the mold from which it was cast, then heating the preform, and pulling the preform to the required size of the microlens.